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Ecologists: Forest fire frequency in Yellowstone ecosystem may increase because of warming climate

August 10, 2011

Climate change in the greater Yellowstone ecosystem will increase the frequency of large forest fires, potentially altering the area’s vegetation and ecology by 2050, according to a team of ecologists who modeled the effects of higher temperatures on fire occurrence. The changes could result in a shift from mature and old-growth forests to younger forests and woodlands.

The greater Yellowstone ecosystem encompasses about 20 million acres in Wyoming, Montana and Idaho and includes Yellowstone National Park, Grand Teton National Park, national forests and private land. The forests in this area are predominantly lodgepole pine and include Douglas fir, ponderosa pine, whitebark pine, spruce-fir and aspen.

“Large, severe fires are normal for this ecosystem and have occurred about every one to three centuries over the past 10,000 years,” said Bill Romme, professor emeritus at Colorado State University.

“But if the climate continues changing as projected and the current relationship between climate and large fires holds true, the warming climate will drive far more frequent large fires in the future than have occurred historically.”

Increased temperatures = increased forest fires
Wildfires in the Yellowstone ecosystem are primarily dependent on temperature, relative humidity and drought conditions. Fire ecologists have already noticed increased fire frequency in the West, associated with temperature increases of less than 2 degrees Fahrenheit and early spring snowmelt in the mountains.

Researchers for the Yellowstone ecosystem study (whose findings were published in the Proceedings of the National Academy of Sciences) analyzed wildfire and climate data in the northern Rocky Mountains from 1972 to 1999, then combined the observed relationships with the trends predicted by climate models to project how climate change will likely impact fires during the 21st century.

They found that large fires (greater than 500 acres in size) will likely occur almost every year within the greater Yellowstone ecosystem by 2050.

Additionally, the fire rotation — the time span over which an area the size of the entire landscape burns — may be reduced from a historical range of 100 to 300 years to less than 30 years.

In simulations conducted by the researchers, years with no major fires (which are common historically) became rare approaching 2050 and almost non-existent between 2050 and 2099. Between 2005 and 2034, the fire interval drops below 30 years in parts of the landscape, and by 2099 climatic conditions are such that fire is the norm.

The predicted new fire regime resembles patterns more typical of warmer and drier landscapes, such as the ponderosa pine forests of the southwest.

Shift in forest vegetation?
Under the researchers’ scenario, the lodgepole pines that dominate much of the Yellowstone-area landscape may not have time to recover between big fires, and forests could shift toward fast-growing aspen and fire-tolerant Douglas-fir, or shrubs and grassland in some places.

Such changes would affect the region’s wildlife, hydrology, carbon storage and aesthetics.

Complicating the predictive nature of the ecosystem as climate changes is the fact that models used in the study will not work once the increase in fires creates a fundamental change in the ecosystem. In other words, as fires increase and the landscape changes, the relationships between climate and fire will change as well.

For example, if projections for winter snow pack or summer rainfall were to change significantly, the study’s results would be impacted.

Eventually, the availability of fuels for fires will become the limiting factor for fires rather than climate. If this occurs, all of the models break down and the future becomes more unpredictable.